This invention relates to plastic additives which are useful as nucleating agents and which are especially useful for improving the optical properties of polymeric materials. More particularly, this invention relates to certain symmetric DBS compounds comprising specific pendant groups, such as C3-C6 alkyl, C1-C6 alkoxy, nitro, as well as phenyl and methylenedioxy(as the combination of two available sites on the pertinent ring system), and furthermore the benzylidene rings may be indan or tetralin. Because of the required symmetrical configuration, the pendant groups on each ring system of the dibenzylidene sorbitol compound must be located at the same positions. Such compounds may be added to or incorporated within polymer compositions which may then be utilized within, as merely examples, food or cosmetic containers and packaging.
All U.S. Patents cited below are herein entirely incorporated by reference.
Numerous attempts have been made to improve the clarity and physical properties of polyolefins through the incorporation of certain kinds of additives. Certain applications require good clarity or transparency characteristics. These include certain types of plastic plates, sheets, films, containers, and syringes that need to exhibit clarity primarily to facilitate identification of articles, etc., stored, wrapped, and/or covered therewith. Such commercially available plastic additives fall into two categories termed xe2x80x9cmelt sensitivexe2x80x9d and xe2x80x9cmelt insensitivexe2x80x9d. Melt sensitive additives possess melting points below or near the normal processing temperatures of polyolefin-based resins and include dibenzylidene sorbitol (DBS) systems. Melt insensitive additives do not melt at normal processing temperatures and include sodium benzoate and salts of organic phosphates as examples.
U.S. Pat. No 4,016,118 to Hamada, et al. teaches that a polyolefin plastic composition containing 0.1% to 0.7% dibenzylidene sorbitol (DBS) as an additive will show improved transparency and reduced molding shrinkage over compositions containing a substituted benzoic acid salt. Additional advancements in sorbitol-based clarification technology have been driven by the need for improved transparency, reduction of plate-out during processing, and improved organoleptic properties (e.g., odor, taste, etc.). In order to overcome these deficiencies, many derivatives of DBS in which the aromatic rings are substituted with various groups have been proposed.
Mahaffey, in U.S. Pat. No. 4,371,645 discloses a series of dibenzylidene sorbitols having the general formula: 
wherein R, R1, R2, R3, and R4, are selected from hydrogen, lower alkyl, hydroxy, methoxy, mono- and di-alkylamino, amino, nitro, and halogen, with the proviso that at least one of R1, R2, R3, and R4 is chlorine or bromine. Effective concentrations of the disclosed substituted DBS derivatives range from 0.01 to about 2 percent of the total composition by weight. Further improvements in transparency characteristics are disclosed by Titus, et al. in U.S. Pat. No. 4,808,650. In this patent mono and disubstituted DBS derivatives having the formula: 
in which R may be hydrogen or fluorine provide improved clarity applications in polyolefins. Rekers, in U.S. Pat. No. 5,049,605 discloses a series of dibenzylidene sorbitols having the general formula: 
in which R1 and R2 are independently selected from lower alkyl groups containing 1-4 carbons which together form a carbocyclic ring containing up to 5 carbon atoms. Also disclosed are polyolefin plastics containing the above group of dibenzylidene sorbitols. Videau, in U.S. Pat. No. 5,696,186 discloses substituted DBS derivatives with an alkyl group (methyl, ethyl, or the like) or halogen (fluorine, chlorine, or the like) on the benzene rings for use as nucleation/clarification agents in polyolefins.
Dibenzylidene sorbitol (DBS) is a well known gelling agent for a variety solvent systems as disclosed in U.S. Pat. No. 4,154,816, Roehl et al.; U.S. Pat. No. 4,816,261, Luebbe et al.; and U.S. Pat. No. 4,743,444 to McCall. U.S. Pat. No. 5,609,855 to Oh et al. and PCT Patent Application WO/92/19221 to Juneja et al.; disclose that di(meta-fluorobenzylidene) sorbitol and di(meta-chlorobenzylidene) sorbitol are extremely useful as gelling agents in the preparation of antiperspirant gel sticks. These two respective DBS systems form effective hard gels and show improved gel stability in the acidic environment of antiperspirant formulations.
According to the present invention, a polyolefin plastic composition having improved transparency is provided which comprises a polymer selected from aliphatic polyolefins and copolymers containing at least one aliphatic olefin and one or more ethylenically unsaturated comonomers and at least one mono-, di-, or tri-acetal which is the reaction product of at least one mole of alditol (such as sorbitol, xylitol, ribitol, and the like) and at least one mole of a benzaldehyde selected from the compounds conforming with Formula (I) 
wherein R1, R2, R3, R4, and R5 are independently selected from the group consisting of hydrogen, alkyl groups containing 3-6 carbon atoms, alkoxy groups containing 1-6 carbon atoms, and phenyl, or any two adjacent groups may be combined to form a cyclic group, wherein said cyclic group is selected from the group consisting of methylenedioxy, cyclopentyl, and cyclohexyl; with the proviso that at least one group of R1, R2, R3, R4, and R5 is a group other than hydrogen. Preferably, such a reaction product is a di-acetal (and thus the result of a 1:2 molar ratio reaction between the alditol and benzaldehyde), and particularly where the alditol is sorbitol, said di-acetal reaction product conforming to the structure of Formula (II): 
wherein p is 0, 1, or 2, R1, R2, R3, R4, and R5 are independently selected from the group consisting of hydrogen, alkyl groups containing 3-6 carbon atoms, alkoxy groups containing 1-6 carbon atoms, or any two adjacent groups may be combined to form methylenedioxy; with the proviso that at least one group of R1, R2, R3, R4, and R5 is a group other than hydrogen.
It should be appreciated with regard to the structural formula set forth above that while only the 1,3:2,4 isomer is represented, this structure is provided for convenience only and the invention is not limited to only isomers of the 1,3:2,4 type, but may include any and all other isomers as well so long as the compound contains two aldehyde substitutents on the alditol moiety.
Throughout this specification, the term xe2x80x9csymmetricalxe2x80x9d as it pertains to di- or tri-acetals of alditols is intended to mean wherein such alditol acetals that possess all acetal linkages (such as 1,3- and 2,4- for di-acetals) derived from the same benzaldehyde.
The diacetals, triacetals, and monoacetals of the present invention are condensation products of alditol, such as sorbitol or xylitol, and specific substituted benzaldehydes (including arylaldehydes). In accordance with this invention, specific examples of suitable substituted benzaldehydes include 4-t-butylbenzaldehyde, 4-isopropylbenzaldehyde, 3,4-methylene-dioxybenzaldehyde, 3,4-dimethoxybenzaldehyde, 3,4-diethoxybenzaldehyde, and the like, to provide the required symmetrical compounds in reaction with an alditol (such as sorbitol, xylitol, ribitol, and the like). Other suitable substituted benzaldehydes for this inventive compounds include, without limitation, 2,4-diisopropylbenzaldehyde, 2,4-di-t-butylbenzaldehyde, 2,4-dimethoxybenzaldehyde, 2,4,5-trimethoxybenzaldehyde, 2,4-diethoxybenzaldehyde, 4-n-pentylbenzaldehyde, 3-methyl-4-methoxybenzaldehyde, 4-methoxy-2,3-dimethylbenzaldehyde, 3-methoxy-2,4-dimethylbenzaldehyde, 2,4-dimethoxy-3-methylbenzaldehyde, 4-ethoxy-3,5-dimethylbenzaldehyde, and 3-isopropyl-4-methoxybenzaldehyde, and the like. Preferred di-acetals of the present invention include 1,3:2,4-bis(4-t-butylbenzylidene) sorbitol and 1,3:2,4-bis(3,4-methylenedioxybenzylidene) sorbitol.
The di-acetals of the present invention may be prepared by a variety of techniques, some of which are known in the art. Generally, such procedures employ the reaction of one mole of D-sorbitol with about 2 moles of aldehyde (for diacetals) in the presence of an acid catalyst (of course, to produce triacetals a 1:3 molar ratio should be followed; for monoacetals, 1:1 ratios are necessary). The temperature employed in the reaction will vary widely depending upon the characteristics, such as melting point, of the aldehyde or aldehydes employed as a starting material in the reaction. The reaction medium may be an aqueous medium or a non-aqueous medium. One very advantageous method that can be employed to prepare di-acetals of the invention is described in U.S. Pat. No. 3,721,682, to Murai et al. (New Japan Chemical Company Limited), the disclosure of which is hereby incorporated herein by reference. While the disclosure of the patent is limited to benzylidene sorbitols, it has been found that the di-acetals of the present invention may also be conveniently prepared by the method described therein. Additional methods for preparing DBS systems can be found in U.S. Pat. No. 5,731,474 to Scrivens et al., U.S. Pat. No. 4,902,807 to Kobayashi et al. which discloses DBS having an alkyl group or halogen for use as clarifying agents, and U.S. Pat. No. 5,106,999 to Gardlik et al. which discloses the preparation of di(meta-fluorobenzylidene) sorbitol, di(meta-chlorobenzylidene) sorbitol, and di(meta-bromobenzylidene) sorbitol.
The inventive sorbitol di-acetals prepared by the above techniques may contain minor impurities (triacetals or monoacetals, for example). Although it may not always be necessary to remove these impurities (particularly if they are present in very low proportions) prior to incorporation of the di-acetal into the target polyolefin, it may be desirable to do so and such purification may serve to enhance the transparency of the resin produced thereby. Purification of the di-acetal may be accomplished, for instance, by removal of the tri-acetal or mono-acetal impurities by the extraction thereof with a relatively non-polar solvent. By removal of the impurities, the product may be purified so that the amount of di-acetal in the additive composition contains at least about 90 percent and even up to 95 percent di-acetal or more.
The proportion of di-acetal in the composition of this invention is an amount sufficient to improve the transparency of the composition, generally from about 0.01 to about 2 percent by weight, preferably about 0.1 to about 1 percent by weight, based upon the total weight of the composition may be provided. When the content of the di-acetal is less than about 0.01 percent by weight, the resulting composition may not be sufficiently improved in respect to transparency characteristics. When the content of di-acetal is increased beyond about 2 percent by weight, no additional advantage can be observed.
The polyolefin polymers of the present invention may include aliphatic polyolefins and copolymers made from at least one aliphatic olefin and one or more ethylenically unsaturated comonomers. Generally, the comonomers, if present, constitute a minor amount, e.g., about 10 percent or less or even about 5 percent or less, of the entire polyolefin, based upon the total weight of the polyolefin. Such comonomers may serve to assist in clarity improvement of the polyolefin, or they may function to improve other properties of the polymer. Examples include acrylic acid and vinyl acetate, etc. Examples of olefin polymers whose transparency can be improved conveniently according to the present invention are polymers and copolymers of aliphatic monoolefins containing 2 to about 6 carbon atoms which have an average molecular weight of from about 10,000 to about 2,000,000, preferably from about 30,000 to about 300,000, such as polyethylene, linear low density polyethylene, polypropylene, crystalline ethylenepropylene copolymer, poly(1-butene), 1-hexene, 1-octene, vinyl cyclohexane, and polymethylpentene. The polyolefins of the present invention may be described as basically linear, regular polymers that may optionally contain side chains such as are found, for instance, in conventional, low density polyethylene.
Other polymers that may benefit from the nucleation and clarification properties of the sorbitol acetals of the present invention include polyethylene terephthalate, polybutylene terephthalate, and polyamides, among others.
The olefin polymer or copolymer used in the composition of the present invention is crystalline, and the diffraction of light caused by micro crystals contained in it is considered to be responsible for the deterioration of the transparency of the polymer. It is thought that the di-acetal functions in the composition to reduce the size of the microcrystals thereby improving the transparency of the polymer.
The composition of the present invention can be obtained by adding a specific amount of the di-acetal directly to the olefin polymer or copolymer, and merely mixing them by an suitable means. Alternatively, a concentrate containing as much as about 20 percent by weight of the di-acetal in a polyolefin masterbatch may be prepared and be subsequently mixed with the resin. Furthermore, the inventive alditol derivatives (and other additives) may be present in any type of standard polyolefin additive form, including, without limitation, powder, prill, agglomerate, liquid suspension, and the like, particularly comprising dispersion aids such as polyolefin (e.g., polyethylene) waxes, stearate esters of glycerin, montan waxes, mineral oil, and the like. Basically, any form may be exhibited by such a combination or composition including such combination made from blending, agglomeration, compaction, and/or extrusion.
Other additives such as a transparent coloring agent or plasticizers (e.g., dioctyl phthalate, dibutyl phthalate, dioctyl sebacate, mineral oil, or dioctyl adipate), can be added to the composition of the present invention so long as they do not adversely affect the improvement of transparency of the product. It has been found that plasticizers such as those exemplified above may in fact aid in the improvement of the transparency by the di-acetal.
With regard to other additives it may also be desirable to employ the di-acetals, triacetals, or monoacetals disclosed above in combination with other conventional additives having known transparency improving effects such as, for instance, para-t-butylbenzoic acid, its salts, low molecular weight waxy polypropylene and the like. It may even be desirable to provide the particular di-acetals of the present invention in the polyolefin composition in combination with the previously described dibenzylidene sorbitol additive disclosed in U.S. Pat. No. 4,016,118 to Hamada et al. In such applications, generally at least about 10 percent, preferably about 25 percent, or even about 50 percent or more of the clarity improving component will be the diacetals of the present invention, with the remainder being comprised of other known clarifying agents, plasticizers, etc.
The compositions of the present invention may be obtained by adding the inventive symmetric substituted benzylidene sorbitol acetal to the polymer or copolymer and merely mixing the resultant composition by any suitable means. The composition may then be processed and fabricated by any number of different techniques, including, without limitation, injection molding, injection blow molding, injection stretch blow molding, injection rotational molding, extrusion, extrusion blow molding, sheet extrusion, film extrusion, cast film extrusion, foam extrusion, thermoforming (such as into films, blown-films, biaxially oriented films), thin wall injection molding, and the like into a fabricated article.
Other additives may also be used in the composition of the present invention, provided they do not interfere with the primary benefits of the invention. It may even be advantageous to premix these additives or similar structures with the nucleating agent in order to reduce its melting point and thereby enhance dispersion and distribution during melt processing. Of particular interest is the incorporation of the inventive symmetrical compound or compounds with, without limitation to any specific additive nucleators or clarifiers, selected amounts of bis(3,4-dimethylbenzylidene) sorbitol (hereinafter DMDBS). As noted below, such a combination provides unexpected haze benefits within target polyolefin (e.g., polypropylene) plastic articles. Other additives well known to those skilled in the art may be present as well, including, without limitation, plasticizers, lubricants, catalyst neutralizers, antioxidants, light stabilizers, colorants, other nucleating agents, and the like. Some of these additives may provide further beneficial property enhancements, including improved aesthetics, easier processing, and improved stability to processing or end use conditions.
In particular, it is contemplated that certain organoleptic improvement additives be added for the purpose of reducing the migration of degraded benzaldehydes from reaching the surface of the desired article. The term xe2x80x9corganoleptic improvement additivexe2x80x9d is intended to encompass such compounds and formulations as antioxidants (to prevent degradation of both the polyolefin and possibly the target alditol derivatives present within such polyolefin), acid neutralizers (to prevent the ability of appreciable amounts of residual acids from attacking the alditol derivatives), and benzaldehyde scavengers (such as hydrazides, hydrazines, and the like, to prevent the migration of foul tasting and smelling benzaldehydes to the target polyolefin surface). Such compounds and formulations can be added in any amounts in order to provide such organoleptic improvements as needed. However, the amounts should not appreciably affect the haze results for the target polyolefin itself. Thus, lower amounts on the order of from about 20 ppm to about 2,000 ppm of the total polyolefin component are desired.
The compositions of the present invention are suitable as additives to improve the clarity of packaging materials and container materials for cosmetics, food-stuffs, and the like, because they give film, sheet, and other fabricated articles having excellent transparency and physical properties.